Filtering of molten metal

ABSTRACT

The disclosure teaches an improvement in the filtration of molten metal, especially aluminum, using a removable filter plate. The filter plate is provided with a bevelled peripheral surface mating with a like bevelled surface in a filter chamber so that the filter plate can be conveniently inserted in the chamber and removed therefrom.

This is a Division of application Ser. No. 597,963, filed July 21, 1975,now U.S. Pat. No. 4,024,056.

BACKGROUND OF THE INVENTION

Molten metal, particularly molten aluminum, in practice generallycontains entrained solids which are deleterious to the final cast metalproduct. These entrained solids appear as inclusions in the final castproduct after the molten metal is solidified and cause the final productto be less ductile or to have poor bright finishing and anodizingcharacteristics. The inclusions may originate from several sources. Forexample, the inclusions may originate from surface oxide films whichbecome broken up and are entrained in the molten metal. In addition, theinclusions may originate as insoluble impurities, such as carbides,borides and others or eroded furnace and trough refractories.

It is naturally highly desirable to improve the filtration of moltenmetals in order to remove or minimize entrained solids in the final castproduct, particularly with respect to molten aluminum and especially,for example, when the resultant metal is to be used in a decorativeproduct, such as decorative trim or sheet made from the 5000 series ofaluminum alloys, as aluminum Alloys 5252 and 5657. Inclusions asaforesaid cause loss of properties in the final solidified alloy andlead to degradation of processing efficiency and loss of properties inthe final product. For example, one type of finishing flaw which isparticularly significant in decorative trim or sheet made from aluminumAlloy 5252 is a stringer defect known as a linear defect.

Rigorous melt treatment processes, such as gas fluxing, minimize theoccurrence of such defects; however, these are not successful inreducing them to a satisfactory level for critical applications.Conventionally, melt filtration is utilized in order to decrease theextent of such defects, and others caused by the presence of inclusionsin the melt. The most common form of melt filtration involves the use ofopen weave glass cloth screens placed in transfer and pouring troughs orin the molten pool of metal in the top of a solidifying ingot. Suchfilters have been found to be only partially effective since they removeonly the larger inclusions. Another type of filter in common use is abed filter made up, for example, of tabular alumina. Such filters havemany disadvantages, perhaps the most serious of which is the greatdifficulty experienced in controlling and maintaining the pore sizenecessary for efficient filtration. Another difficulty with such filtersis their tendency to produce an initial quantity of metal having poorquality at the start up of each successive casting run. This behaviorresults in a so-called ingot "butt effect", that is, ingots having buttportions of relatively poor quality which must be scrapped and recycled.Still further, the metal in a bed filter must be maintained molten evenwhen the filter is not in use.

Porous ceramic foam materials are known in the art, for example, havingbeen described in U.S. Pat. Nos. 3,090,094 and 3,097,930. These porousceramic foam materials are known to be particularly useful in filteringmolten metal, as described in U.S. Pat. No. 3,893,917 for "Molten MetalFilter" by Michael J. Pryor and Thomas J. Gray, patented July 8, 1975,and also as described in copending U.S. patent application Ser. No.563,213 for "Ceramic Foam Filter" by John C. Yarwood, James E. Dore andRobert K. Preuss, filed Mar. 28, 1975.

Porous ceramic foam materials are particularly useful for filteringmolten metal for a variety of reasons included among which are theirexcellent filtration efficiency, low cost, ease of use and ability touse same on a disposable, throwaway basis. The fact that these ceramicfoam filters are convenient and inexpensive to prepare and may be usedon a throwaway basis requires the development of means for easily andconveniently assembling and removing porous, molten metal filters from afiltration unit while providing a highly efficient filtration assembly.

Accordingly, it is a principal object of the present invention toprovide an improved method and apparatus for the filtration of moltenmetal with a removable filter plate.

It is a particular object of the present invention to provide animproved removable filter plate for use in the filtration of moltenmetal.

It is a still further object of the present invention to provideimprovements as aforesaid which are convenient and inexpensive toutilize and which result in high filtration efficiency.

Further objects and advantages of the present invention will appearhereinbelow.

SUMMARY OF THE INVENTION

In accordance with the present invention it has been found that theforegoing objects and advantages may be readily obtained.

The present invention provides a highly efficient filtration assemblywhich utilizes a conveniently removable filter plate. The removablefilter plate of the present invention is easily assembled anddisassembled in the filtration assembly and enables one to obtainexcellent filtration efficiency.

In accordance with the method of the present invention a filter chamberis provided having a metal inlet and a metal outlet and having abevelled wall surface adapted to be partitioned by a filter plate; arigid filter plate is provided for filtration of molten metal having anopen cell structure characterized by a plurality of interconnectedvoids, preferably a ceramic foam filter plate with the interconnectedvoids being surrounded by a web of said ceramic, wherein the filterplate has a bevelled peripheral surface adapted to mate with thebevelled wall surface of the filter chamber; and a resilient sealingmeans is provided on said bevelled filter plate surface resistant tosaid molten metal. The filter plate and sealing means are inserted inthe filter chamber to engage the bevelled wall surface of the filterchamber and thereafter molten metal is passed through the filter platefor discharge through said outlet, wherein the surface of the filterplate is maintained below the level of the molten metal.

Accordingly, it can be also seen that the present invention provides animproved filter plate for use in the filtration of molten metal havingan open cell structure characterized by a plurality of interconnectedvoids, preferably a ceramic foam filter as aforesaid, said plate havinga bevelled peripheral surface adapted to mate with a correspondingbevelled surface in a filter chamber. The present invention alsoprovides an improved filtration apparatus comprising: a filter chamberhaving a metal inlet and metal outlet and having a bevelled wall surfaceadapted to be partitioned by a filter plate; a filter plate forfiltration of molten metal having an open cell structure characterizedby a plurality of interconnected voids, said plate having a bevelledperipheral surface mating with the bevelled wall surface of the filterchamber; and a resilient sealing means between and engaging said matingsurfaces resistant to said molten metal, wherein the filter platepartitions the filter chamber so as to be readily insertable therein andremovable therefrom.

As indicated hereinabove, the present invention provides considerableadvantages in the filtration of molten metal, especially aluminum. Thus,for example, the present invention enables one to filter molten metalwith a conveniently removable filter plate which may be easily andquickly inserted in the filtration apparatus and easily and convenientlyremoved therefrom. In accordance with the preferred embodiment of thepresent invention when a ceramic foam filter plate is utilized,extremely high filtration efficiencies are obtained and theseefficiencies are obtained utilizing a disposable filter plate which canbe easily and conveniently inserted and removed from the filtrationapparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of the filter chamber of the present inventionincluding the filter plate in place therein substantially horizontallydisposed;

FIG. 2 is a sectional view along the lines II--II of FIG. 1;

FIG. 3A is a perspective view of the filter plate of the presentinvention as shown in FIGS. 1 and 2;

FIG. 3B is a perspective view of a modified filter plate similar to thefilter plate shown in FIG. 3A;

FIG. 4 is a top view of another embodiment of the filter chamber of thepresent invention wherein the filter plate is substantially verticallydisposed;

FIG. 5 is a sectional view along the lines V--V of FIG. 4;

FIG. 6 is a perspective view of the filter plate of the presentinvention as shown in FIGS. 4 and 5; and

FIG. 7 is a sectional view of another embodiment of the presentinvention wherein the filter plate is disposed above an individualpouring spout.

DETAILED DESCRIPTION

A filter chamber is illustrated in FIGS. 1 and 2, as in a molten metaltransfer system, pouring pans, pouring troughs, transfer troughs, metaltreatment bays, or the like. The filter apparatus 2 may if desired beconstructed in two sections 2a and 2b which may be bolted together byany suitable means, such as by flanges at the peripheries thereof, notshown. The particular filter apparatus illustrated in FIGS. 1 and 2 is atransfer trough containing a central filter chamber 3 fed by inlet 4with the metal passing out of the filter chamber via outlet 5. Themolten metal may enter the inlet 4 by any suitable means, such aspouring spout 6. The filter chamber 3 is a bowl shaped chamber, thebottom of which is recessed below the level of the inlet 4 so that themolten metal passing into the filter chamber 3 may travel downwardlythrough the filter plate of the present invention in place in the filterchamber. Thus, the filter chamber 3 is characterized by a peripheral rim7 which may completely surround the upper portion of the filter chamber.As shown in FIG. 1, the filter chamber rim 7 surrounds the filterchamber on all sides except adjacent the area of inlet 4. The filterchamber rim 7 is connected to side wall 8 which extends downwardly tofilter chamber floor 9 which has a circumferential bevelled portion oraperture 10 (FIG. 2) extending around the periphery thereof to mate withthe bevelled wall surface of the filter plate. The filter plate 11 has acorresponding bevelled peripheral surface 12 adapted to mate with thebevelled wall surface 10 of the filter chamber. The bevelled peripheralsurface of filter 12 is provided with a resilient sealing means 13thereon resistant to the molten metal, and the filter plate 11 andsealing means 13 are inserted in the filter chamber 3 so that the filterplate-sealing means assembly engages the bevelled wall surface of thefilter chamber.

Thus, as shown in FIGS. 1 and 2, the filter 11 is substantiallyhorizontally disposed in a trough. The filter as shown has a squareconfiguration; however, any convenient shape may be readily employed forthe filter, such as round, hexagonal or the like. The filter plate 11 ispositioned in a recessed section of a filter chamber or filter bowl 3,such as in the floor portion 9 thereof. Molten metal is fed to thefilter 11 via inlet 4 into the filter chamber 3. The molten metal passesdownwardly through filter 11 into the recess 14 beneath the filter plate11. The filter 11 is sealed in place by means of resilient seal 13 sothat the filter plate may be readily inserted by pressure verticallydownwards and easily removed by pressure vertically upwards.Alternatively, as indicated above, the filter chamber may be split andmoved laterally for positioning the gasketed or sealed filter plate inthe filter chamber, with the filter plate held therein with a vise typeaction. Preferably, the bevelled peripheral surface of the filterchamber 10 is bevelled at an angle of from 2° to 20° and the filterplate is preferably provided with a bevelled surface 12 correspondingthereto at an angle of from 2° to 20°. The filter plate 11 is preferablysubstantially horizontally disposed at an angle of from 1° to 5°upwardly sloped towards the metal outlet 5 in order to prevententrapment of air against the underside of the filter. In addition, thefloor 15 of recess 14 beneath filter plate 11 is preferably slopeddownwardly at an angle of from 1° to 5° sloped towards outlet 5 in orderto facilitate drainage of metal during operation and at the completionof the pouring or transfer operation.

Alternatively, if desired, the filter chamber may be split horizontally,as along a horizontal plane beneath floor 9, or on an angle beneathfloor 9, particularly to enable easy cleaning of recess 14. It may bedesirable to reverse the direction of the taper of bevelled portion 10,effecting a positive seal by means of the wall portion in recess 14.

Filters up to several inches thick and several square feet in area canbe readily and conveniently located in pouring or transfer troughs orthe like as described above. It is preferred, however, to restrict thethickness of the filter plate to from 1/2 inch to 4 inches. Conventionalmaterials of construction may be utilized in the preparation of thetransfer trough including the filter chamber 3 and inlet and outlets 4and 5, such as, for example, making the filter chamber and inlet andoutlets of castable refractory or ceramic tile.

The velocity of the molten metal below the filter is determined by theflow rate of the metal and the cross section of the flow channel underthe filter. This velocity should be high enough to help sweep airbubbles away. Preferably the filter should be level with or slightlybelow the bottom surface of floor portion 9 to avoid catching airbubbles thereon. Preferably also the top surface of the filter should belevel or slightly recessed to avoid the formation of a metal dam. Thedepth of the chamber above the filter should be sufficient to ensurepriming of the filter without overflow, as, for example, preferably adepth of at least 6 inches and generally higher than 10 inches.

The filter plate of the present invention should have an open cellstructure characterized by a plurality of interconnected voids so thatthe molten metal passes therethrough for use in removing or minimizingentrained solids from the final cast product, as, for example, a solidfilter plate made from sintered ceramic aggregate or a porous carbonplate. In the preferred embodiment, a ceramic foam filter is utilized asdescribed in the aforesaid copending application Ser. No. 563,213. Inaccordance with the teaching of said copending application, the ceramicfoam filter has an open cell structure characterized by a plurality ofinterconnected voids surrounded by a web of said ceramic material. Theceramic filter has an air permeability in the range of from 400 to 8,000× 10⁻⁷ cm², preferably from 400 to 2500 × 10⁻⁷ cm², a porosity or voidfraction of 0.80 to 0.95 and from 5 to 45 pores per linear inch,preferably from 20 to 45 pores per linear inch. The molten metal flowrate through the filter should be from 5 to 50 cubic inches per squareinch of filter area per minute. The ceramic foam filter described insaid Ser. No. 563,213 is particularly suitable in the present inventionsince it is low cost and may be readily employed on a throwaway basis.Furthermore, this filter is surprisingly effective in the filtration ofmolten metal, especially aluminum, at a low cost achieving surprisingfiltration efficiency with considerable flexibility.

The ceramic foam filter preferably utilized in the present invention isprepared from an open cell, flexible foam material having a plurality ofinterconnected voids surrounded by a web of said flexible foam material,such as polyurethane foams or cellulosic foams. The ceramic foam filtermay be prepared in accordance with the general procedure outlined inU.S. Pat. No. 3,893,917 wherein an aqueous ceramic slurry is preparedand the foam material impregnated therewith so that the web thereof iscoated therewith and the voids substantially filled therewith. Theimpregnated material is compressed so that a portion of the slurry isexpelled therefrom and the balance uniformly distributed throughout thefoam material. The coated foam material is then dried and heated tofirst burn out the flexible organic foam and then sinter the ceramiccoating, thereby providing a fused ceramic foam having a plurality ofinterconnected voids surrounded by a web of bonded or fused ceramic inthe configuration of the flexible foam. Naturally, a wide variety ofceramic materials may be chosen depending upon the particular metal tobe filtered. Preferably, a mixture of alumina and chromia is employed;however, these materials may naturally be utilized separately or incombination with other ceramic materials. Other typical ceramicmaterials which may be employed include zirconia, magnesia, titaniumdioxide, silica and mixtures thereof. Normally, the slurry contains fromabout 10 to 40% of water and one or more rheological agents, binders, orair setting agents.

As shown in FIG. 3A, the filter plate of the present invention 11 has abevelled peripheral surface 12 adapted to mate with a bevelled surface10 of the filter chamber. Naturally, variations in design arecontemplated within the scope of the present invention, such as shown inFIG. 3B wherein a corresponding flat surface 16 is provided around theentire periphery of filter plate 11 adjacent bevelled surface 12. FIGS.3A and 3B show filter plates wherein the bevelled surface extends aroundthe entire periphery of the plate; however, it may be more convenient toprovide that the bevelled surface extends around less than the entireperiphery, as in FIG. 6 wherein the bevelled surface extends around twofaces of the plate.

Thus, it can be seen that the filter plate of the present invention maybe conveniently utilized in a variety of locations, including pouringpans, pouring troughs, transfer troughs, pouring spouts and metaltreatment bays. The filter should not be placed in the immediatevicinity of turbulent molten metal flow, especially where suchturbulence results in oxide formation and entrainment. This is true forthe case of turbulence both upstream and downstream of the filter.Turbulence upstream of the filter with attendant oxide entrainment tendsto lead to channeling of the filter, inefficient filtration and insevere cases premature blockage of the filter. Turbulence downstream ofthe filter will tend to undo the good rendered by the filter and onceagain load the molten metal with oxide or other nonmetallics which arepresent or are formed on the surface of the metal. Frequentlyencountered sources of turbulence are furnace tap holes, pouring spoutsand other devices which cause rapid changes in flow cross section andconsequent high velocity gradients. Naturally, the particular filterinstallation must be chosen with care to ensure that it too does notbecome another source of turbulence. The foregoing considerations ofturbulence are, of course, particularly relevant to chemically reactivemetals, such as aluminum and magnesium and their alloys which readilyoxidize on contact with air; however, these considerations are alsosignificant for less reactive metals, such as copper and its alloys.Naturally, devices are available in the art to mitigate turbulence, as,for example, appropriately placed vanes.

As can be seen from the foregoing, the filter plate of the presentinvention may be conveniently disposed substantially horizontally. Inaddition, if desired, the filter may be disposable substantiallyvertically or at an intermediate angle with respect to the flow ofmolten metal. The vertical disposition of the filter plate of thepresent invention will be described in a specific embodimenthereinbelow. The horizontally disposed filter, however, has twoprincipal advantages over a vertically disposed filter. First, byutilizing the significant length of a pouring trough or length andbreadth of a pouring pan a large filter area can be easily andconveniently accommodated. On the other hand, in order to accommodate alarge filter area in a vertical installation, one must generally resortto a deep trough, pan or tundish. Such geometric constraints are oftenof great practical significance, especially where adaptation of existingcasting facilities is contemplated. A second advantage of a horizontalfilter is the fact that it ensures that all parts of the filter willprime under substantially the same metallostatic head; whereas, avertical filter will naturally prime under a head varying from top tobottom. For this reason naturally a vertical filter primes in anon-uniform manner. The upper portions of a vertical filter will notpass as much metal as the lower portions thereof and in fact may notprime at all. Furthermore, changes of head upstream of the filter willhave more effect on a vertical filter than on a horizontal filter andmay result in momentary or significant loss of prime to the upperportions of the filter and even freezing off thereof. To minimize lossof primed area in operation, a filter should remain buried under aminimum head of molten metal. This is easier to achieve with ahorizontal filter than with a vertical one. Also, exposure of unprimedareas of a vertical filter above the metal line can result in crackingof the filter due to thermal stress from the high temperature gradientsobtaining under such conditions. For the foregoing reasons, ahorizontally or substantially horizontally disposed filter is preferredin the present invention.

A disadvantage, however, of horizontally placed filters is that air canbe trapped beneath these filters. This in turn can lead to oxideformation downstream of the filter and to channeling of flow through thefilter and, hence, less than optimum filtration. This type of entrapmentis obviated by a vertical disposition of the filter.

In accordance with the present invention, it has been found that theforegoing disadvantages of horizontally disposed filters can be greatlyminimized while retaining essentially all advantages of a horizontallydisposed filter by canting the filter at a small angle of from 1° to 5°to the horizontal. Such a disposition allows escape of the air duringinitial priming of the filter without relinquishing the uniform orsubstantially uniform depth of immersion of the filter body resultingfrom horizontal placement. Preferably, as shown in FIG. 2, the highpoint on the horizontally disposed filter surface should be at theextreme downstream end of the filter so that the escape of air isaugmented by the sweeping action of the metal stream. It can be seenthat the feature of a substantially horizontally disposed, upwardlysloped filter is highly advantageous so that excellent results may beobtained using said feature without the use of a bevelled peripheralsurface, as, for example, using a split filter chamber and holding thefilter therein with a vise type action. Naturally, an appropriatesealing means should also be used and also the floor 15 of recess 14beneath filter plate 11 is preferably sloped as described above.

It should be noted that before priming one must have sufficient volumeof metal upstream to avoid depletion of the metal feed to the filter andpossible freeze off of portions of the filter.

A wide variety of filter plate configurations can be utilized in thepresent invention depending upon the particular conditions of use andnaturally to allow use with a minimum of modification to existingequipment. Flat plates of any shape are naturally preferred since theyare easier to prepare; however, one may conveniently utilize plugs,curved shapes, hollow cylinders or the like. Generally, a single largefilter unit is employed capable of handling the total flow of metalbeing cast or transferred. However, a number of small filters inparallel may also be used. For example, smaller filter units may bedesigned to fit over individual ingot pouring spouts. In addition, onemay conveniently utilize a single filter of varying porosity or separatefilters of different porosity as described in the aforesaid copendingapplication Ser. No. 563,213 wherein, for example, a larger relativepore size filter is utilized upstream and a smaller relative pore sizeis utilized downstream so that the upstream filter removes grossinclusions and the downstream filter removes smaller entrained solids.

Continuous casting is another instance where a parallel arrangement oftwo or more filters may be advantageously employed. In such anoperation, the great length and associated great total flow of metalinvolved may require changing of filters in mid-run. Such changes may befacilitated by the use of parallel flow channels each containing afiltering unit and a means of diverting flow from one to the other byvalves or dams. Flow would be restricted to one filter and one filterunit at a time and would be diverted to another channel and a freshfiltering unit once the head drop across the first filter unit becameexcessive. It can be seen that such a switching procedure could supplyan endless stream of filter metal to a continuous casting station.Multistage filters can also be used and may be quite advantageous andeven necessary where input metal is of very poor quality in terms ofnonmetallic loading. Such multistage units may consist of two or moreindividual filters in series or may take the form of an array of platespacked firmly or loosely together to form a single filter unit. A singlefilter of continuously graded properties through its thickness may alsobe conveniently employed. Naturally, in this case, the filter units,segments or parts thereof should be made progressively finer frominitial inlet to final outlet location.

Since the filter plate of the present invention is designed to be athrowaway item, it is essential to provide an effective means of sealingthe filter plate in place in its holder which is easy to assemble,disassemble and clean up. The holder or filter chamber itself isnormally an integral part of a trough, pouring pan or tundish, etc. andshould be constructed of refractory materials resistant to the moltenmetal similar to those used in standard trough construction. It isgreatly preferred to seal the filter plate in place using a resilientsealing means or gasket type seal peripherally circumscribing the filterplate at the bevelled portion thereof. The gasket type seals ensure aleak free installation and also provide an effective parting mediumwhich is essential for ease of disassembly. In addition, since thegaskets or sealing means prevent ingress of metal to the sealing facesof the holder unit, their use considerably eases clean up andeffectively prolongs the life of the unit by eliminating problems ofmetal attack. Furthermore, because of its resiliency, the gasket mayprovide sufficient frictional force to hold the filter body in place inthe holder or filter chamber without resort to other types of hold downdevices. The resilient sealing means should be non-wetting to theparticular molten metal, resist chemical attack therefrom and berefractory enough to withstand the high operating temperatures.

Plate type filter units of the present invention may be sealed bygaskets around their edges and/or at the peripheries of their largefaces. The plate type filter units of the present invention arepreferably sealed by an edge type seal along the bevelled peripheralsurface of the filter plate thus providing a positive seal and, inconjunction with the gasket, a mechanical advantage to hold the filterin place. In the event that a simple press fit is insufficient to holdthe filter in place, naturally a variety of mechanical devices such aswedges and hold down weights may be employed. Alternatively, therefractory sealing unit into which the filter is to be fitted can bemade to be split as shown in FIG. 1 so that pressure can be applied tothe seals by the vise like action of closing the split unit. Thebevelled angle of the filter chamber and corresponding bevelled angle ofthe filter plate tends to form a positive seal and hold the filter inplace against buoyancy forces acting thereupon. Naturally, as indicatedabove, the gasket or seal should be resistant to the molten metalutilized. Typical seal materials include fibrous refractory type sealsof a variety of compositions, as the following illustrative seals: (1) aseal containing about 45% alumina, 52% silica, 1.3% ferric oxide and1.7% titania; (2) a seal containing about 55% silica, 40.5% alumina, 4%chromia and 0.5% ferric oxide; and (3) a seal containing about 53%silica, 46% alumina and 1% ferric oxide.

FIGS. 4 and 5 show a vertically disposed filter installation in atransfer trough according to the present invention. In the embodiment ofFIGS. 4 and 5, a filter plate 20 is held in place by a refractory dam 21and positioned in a slot 22 in a filter chamber 23. Molten metal is fedto the filter chamber 23 via inlet trough 24 and passes horizontallyinto filter chamber well 25 and thence through filter plate 20 intooutlet trough 26. Filter plate 20 is sealed into slot 22 by means of aceramic fiber gasket 27 which completely circumscribes the filter plate20. The pregasketed filter plate 20 and dam 21 are placed into the slot22 and sealed in place by means of wedges 28. A drain hole 29 isprovided to drain well 25 of metal at the completion of pouring ortransfer. In operation, the drain hole 29 may be closed by a stopper rodor other convenient closure means, not shown.

The filter plate of the present invention, as filter plate 20, is afrustum or segment of a solid figure with sloping sides so that theperipheral surface thereof has a bevelled configuration. Filter chamberwell 25 has a corresponding bevelled wall surface 30 (FIG. 4) to matewith the bevelled peripheral surface 31 of the filter plate (FIG. 6).Filters up to several inches thick and several square feet in area canbe conveniently located in troughs in the foregoing manner. The dam 21and the filter chamber 23 may be made of conventional materials ofconstruction. The filter well 25 and corresponding trough linings may beconveniently prepared of castable refractory or ceramic tile. The dam 21and wedges 28 may be made of refractory boards such as Marinite if themetal to be filtered is aluminum or some lower melting alloy. Naturally,the sealing means 27 is preferably adjacent the bevelled filter platesurface 31; however, as shown in FIGS. 4-6 where the filter plate isbevelled on only two peripheral faces thereof, the sealing means ispreferably adjacent all peripheral surfaces of the filter plateincluding non-bevelled peripheral surfaces.

FIG. 7 shows an example of a horizontally disposed frustoconical filterplate installation designed to feed a single feeding pouring spout. Inthis unit, filter plate 40 is located in a recess 41 in the refractorybase 42 of a pouring pan or tundish 43. During casting metal from pan 43flows vertically through filter plate 40 into channel 44 beneath filterplate 40 and thence into pouring spout 45 feeding an ingot or castingbelow. The filter plate is provided with a bevelled peripheral surface46 for mating with a corresponding bevelled surface 47 in recess 41. Aresilient sealing means 48 is provided between the correspondingbevelled surfaces so that the presealed filter 40 is positioned andsealed in place by pressure from above in a manner similar to thepreceding embodiments. Preferably, some means should be provided forventing air bubbles from the bottom of the filter.

This invention may be embodied in other forms or carried out in otherways without departing from the spirit or essential characteristicsthereof. The present embodiment is therefore to be considered as in allrespects illustrative and not restrictive, the scope of the inventionbeing indicated by the appended claims, and all changes which comewithin the meaning and range of equivalency are intended to be embracedtherein.

What is claimed is:
 1. In the filtration of molten metal with aremovable filter plate, the method which comprises:providing a filterchamber having a floor, a metal inlet and a metal outlet and having awall surface adapted to be partitioned by a filter plate; providing afilter plate for filtration of molten metal having an open cellstructure characterized by a plurality of interconnected voids, saidplate having a peripheral surface adapted to mate with the wall surfaceof the filter chamber and including a resilient sealing means on saidperipheral filter plate surface resistant to said molten metal;inserting said filter plate and sealing means in said filter chamber toengage the wall surface of the filter chamber so that the filter plateis substantially horizontally disposed at an angle of from 1° to 5°upwardly sloped towards said metal outlet; and thereafter passing moltenmetal through said filter plate for discharge through said outlet,wherein the surface of the filter plate is maintained below the level ofthe molten metal and wherein the floor of the filter chamber beneath thefilter plate is sloped downwardly towards said metal outlet.
 2. Themethod of claim 1 wherein said filter plate is a ceramic foam filterplate having an open cell structure characterized by a plurality ofinterconnected voids surrounded by a web of said ceramic.
 3. The methodof claim 2 wherein said filter has an air permeability in the range of400 to 8,000 × 10⁻⁷ cm², a porosity of 0.80 to 0.95, a pore size of 5 to45 pores per linear inch and a thickness from 178 to 4 inches.
 4. Animproved filtration apparatus comprising: a filter chamber having afloor, a metal inlet and a metal outlet and having a wall surfaceadapted to be partitioned by a filter plate; a filter plate for thefiltration of molten metal having an open cell structure characterizedby a plurality of interconnected voids, said plate having a peripheralsurface mating with the wall surface of the filter chamber, wherein thefilter plate is substantially horizontally disposed at an angle of from1° to 5° upwardly sloped towards said metal outlet; and a resilientsealing means between and engaging said mating surfaces resistant tosaid molten metal, wherein the filter plate partitions the filterchamber so as to be readily insertable therein and removable therefromand wherein the floor of the filter chamber beneath the filter plate issloped downwardly towards said metal outlet.
 5. An apparatus accordingto claim 4 wherein the filter plate is a ceramic foam filter platehaving an open cell structure characterized by a plurality ofinterconnected voids surrounded by a web of said ceramic.
 6. Anapparatus according to claim 5 wherein said filter has an airpermeability in the range of 400 to 8,000 × 10⁻⁷ cm², a porosity of 0.80to 0.95, a pore size of 5 to 45 pores per linear inch and a thicknessfrom 1/2 to 4 inches.